[LVI] Don't require DominatorTree in LVI (NFC)

After D76797 the dominator tree is no longer used in LVI, so we
can remove it as a pass dependency, and also get rid of the
dominator tree enabling/disabling logic in JumpThreading.

Apart from cleaning up the code, this also clarifies LVI
cache consistency, in that the LVI cache can no longer
depend on whether the DT was or wasn't enabled due to
pending DT updates at any given time.

Differential Revision: https://reviews.llvm.org/D76985